An injection molding method has been generally used as the method for making a joint between a main pipe and a branch pipe made of synthetic resin, such as polyethylene. However, the injection molding method has a disadvantage due to manufacturing cost, as different molds are required for pipes having different sizes and shapes, especially when the manufacturing amount is small and requires a specific size. Therefore, in order to reduce the facility cost, other methods have been adopted in which pipes are worked afterwords, which are: the drawing method, the blow molding method and the welding method.
In the drawing method, first, a hole is made in the wall of a main pipe and then the periphery of the hole is softened by heating. Next, a mold having a flat top is inserted into the main pipe from one end thereof, and then the mold is pulled out of the main pipe through the hole by pulling a member which is hooked to the mold. The periphery of the hole rises and a flat top is formed at the periphery of the hole by pulling the mold out of the main pipe. Then, a branch pipe is fixed to the flat top of the periphery of the hole of the main pipe by fitting or cementing. This method has been described in Japanese Patent Provisional Publication No. 62868/73.
In the blow molding method, first, a predetermined part of the wall of a main pipe is softened by heating, and then a hill having a flat top is formed on the softened part, and then a hole having a diameter slightly larger than the outer diameter of a branch pipe is formed. Then, the branch pipe is inserted through the hole and fixed with the main pipe by fitting or cementing. This method has been described in Japanese Utility Model Publication No. 2955/83.
In the welding method, first, a hole is made in the wall of a main pipe, while one end of a branch pipe is cut to a have the corresponding profile of the hole of the main pipe, and then the cut end of the branch pipe and the periphery of the hole are welded. In many cases, the welded peripheries may be coated with fiber-reinforced plastic (FRP) for reinforcing the joint.
However, there are disadvantages to these methods.
In the drawing method, it is required to make a periphery of a flat top having a diameter equal to or larger than the outer diameter of the branch pipe, so this method can not be applied for making a joint between the main and branch pipes having the same diameter, as it is impossible to raise the periphery of the hole of the main pipe.
In the blow molding method, the center of the softened part in the wall of the main pipe is elongated first and then the outer part is elongated gradually to form a flat top of the periphery in blow molding. As a result, the root of the elongated periphery becomes thin, though most of load is concentrated thereto and therefore requires strength. Consequently, the endurance limit of the joint is not satisfied resulting poor reliability.
In the welding method, considerable skill is required for welding, so that the strength in fixing the branch pipe to the main pipe can be stably obtained only by a skilled operator. Further, the welding operation is time-consuming.
These disadvantages may be resolved by adopting the butt-welding method by fusion, in which the two pipe ends of the main and branch pipes are fused by heating and then butted to make a joint under pressure. However, there are disadvantages in the butt-welding method by fusion.
One of the disadvantages in the butt-welding method by fusion, is that it requires the process for removing a bead. A bead is generated in the inner wall of the joint between the main and branch pipes after butt-welding, so it is required that the bead be removed. If a cutting tool such as a rotary cutter or a pressure cutter is used for removing a bead, it is required that the cutting tool be inserted into the main pipe to remove the bead generated in the inner wall thereof and into the branch pipe to remove the bead generated in the inner wall thereof. However, the bead in the branch pipe should be removed by hand instead of the cutting tool, because the inner wall of the branch pipe may be damaged by the cutting tool when inserted deeply into the branch pipe to remove all the bead. Accordingly, the operation becomes complicated, such that the operation consumes time and a stable result can not be obtained. Further, if the pipes are made of crystalline polymer material such, as polyethylene resin, the pipes become deformed to some extent due to stress and relaxation of the material, so that it is required to obtain accuracy in butting the periphery of the hole of the main pipe and the peaked edge of the branch pipe by some means.
If butt-welding is carried out with cores inserted into the pipes from three directions to be located at the inside of the joint as described in Japanese Patent Publication No. 32671/76 and UK Patent No. 1549169, holding members are required for holding the main and branch pipes in three points and supporting members used in inserting and withdrawing the pipes. Additionally, the cores are not always fitted tightly and there may be generated spaces therebetween due to deformation of the main and branch pipes and manufacturing error of the cores and so on, so that there is the possibility of the generation of a bead in the case where fused resin flows into the spaces between the cores.
Second, one of the disadvantages in the butt-welding method by fusion is that appearance of the joint is poor and the strength of the butt-welding is not sufficient. When the main and branch pipes are joined by butt-welding, in which the hole of the main pipe and the peaked edge of the branch pipe are fitted, the fused resin in the periphery of the hole of the main pipe and the peaked edge of the end of the branch pipe is extruded towards the inside and outside of the pipes by applying pressure. As a result, the hole becomes deeper as the fused resin is extruded, even though the hole was made to have the two V-shaped edges located diametrically on the periphery of the main pipe. On the other hand, the peaked edge of the opening of the branch pipe does not change in shape, though the branch pipe becomes short. If the hole of the main pipe becomes deeper, the lowest point of the V-shaped edge of the hole moves lower and becomes narrow, so that more fused resin is extruded out in the vicinity of the lowest point. Consequently, the peaked edge of the opening of the branch pipe is butt-welded deep into the hole of the main pipe and an irregular bead is generated in the butt-welded joint, so that the butt-welded joint has a poor appearance and the fixing strength is not stable due to the difference between the width of the V-shaped edge of the hole of the main pipe and the peaked edge of the opening of the branch pipe.
Third, one of the disadvantages in the butt-welding method by fusion is that a burr is generated in the butt-welding joint. If the branch pipe is fixed to the main pipe such that the axis of the branch pipe is at an angle of 60.degree. with the axis of the main pipe, one of two facets of the hole of the main pipe is required to be formed at an angle of 30.degree. with the axis of the main pipe, so that the inner periphery of the first facet becomes acute. On the other hand, if one of the two facets of the peaked edge of the opening of the branch pipe is required to be formed at an angle of 30.degree. with the axis of the branch pipe, the inner periphery of the peaked edge becomes acute. As a result, the contact area of the first facets of the hole of the main pipe and the peaked edge of the opening of the branch pipe becomes far too large as compared with that of the second facets, so that the pressure to be applied to the joint in butt-welding decreases, especially if the first facets are very acute. Consequently, a bead is generated in the inner peripheries of the facets, as fused resin between the first facets of the hole of the main pipe and the peaked edge of the opening of the branch pipe stays within the inner peripheries, so that a burr is generated at the inner peripheries due to the resin pressure. In order to allow fluid to flow smoothly through the joint, it is required to remove the burr and smooth the inner surface of the pipes.
Next, a problem with the cutting apparatus for cutting pipes used in the method for making a joint between the main and branch pipes will be described. A conventional cutting apparatus for cutting pipes includes a base for fixing a pipe and a cutter, and either the base or the cutter is moved in a straight line. In such a conventional cutting apparatus, there is no difficulty in cutting a pipe in one direction. However, it is required to change the direction of the base versus the pipe to cut the pipe in a different direction after cutting it in one direction when the pipe is to be cut at two different directions in order to obtain a V-shaped edge of a hole or a peaked edge of an opening in the pipe. Therefore, it is required to move the base in an accurate manner to avoid cross cutting of the two cutting facets of different directions or deviation of cutting angle.
Next, problems with the butt-welding apparatus for butt-welding pipes by fusion, used in a method for making a joint between main and branch pipes will be described. One known method in the butt-welding methods by fusion, joins three synthetic resin pipes each of which is cut at an edge at an angle of 45 degrees. In this method, an L-shaped pipe is made by butt-welding two pipes by fusion, at their angular cut edges, and then the corner of the L-shaped pipe is cut at an angle of 45 degrees. Finally, the joint is made by butt-welding the cut opening of the remaining pipe to the cut corner of the L-shaped pipe. When a conventional butt-welding apparatus is used in this method, it is difficult to butt-weld the pipes in an angled direction, because the conventional butt-fusing apparatus is able to butt-fuse pipes only in a predetermined direction, that is a right angle. Additionally, different apparatuses are required for making joints having different shapes, so that the cost becomes high.